The present invention relates to the calibration and test of electrical test probes, and more specifically to a fixture de-embedding method and system for removing test fixture characteristics when calibrating measurement systems.
Electrical signals are the blood that flows through electrical components (e.g. integrated circuits and other electronic devices). Connection mechanisms such as wires, traces, leads, legs, pins, vias, or other connection mechanisms act as the veins and arteries through which the signal blood flows. Electrical test probes (or just “probes”) are used to provide an electrical connection between electrical components and/or connection mechanisms (jointly referred to as “test points” or devices-under-test (DUTs)) and the testing instruments (e.g. oscilloscopes) that measure, monitor, diagnose, and/or process the signals. An electronic test probe generally consists of a probing head, a cable, and a probe connector. The probing head may have an integral or replaceable probing tip that is suitable for making an electronic contact with DUTs. The probing head is attached to a first end of the cable and the connector is attached to the opposite end of the cable. Electrical test probes may be an “active” probe or a “passive” probe. Electrical test probes may be single-ended (having a signal contact and a reference or ground contact) or differential (having two signal contacts). High impedance (microwave) probes are electrical test probes in which the impedance at the probing tip is much higher than the impedance of the DUT at the point of probing tip contact. The term “measurement system” will be used herein to describe the combination of electrical test probes (such as high impedance probes) and testing instruments.
When taking electrical measurements (or otherwise monitoring, diagnosing, and/or processing signals), it is vital that the characteristics of the measurement system do not affect the accuracy of the measured data. Accordingly, the measurement system is ideally designed to be as close to perfect as possible. For example, for high impedance probes that measure voltages at test points, the probe impedance is designed to be high enough so that connection of the probe will make no change to the voltage in the DUT (i.e. the impedance of the probe can be assumed to be insignificant compared to the test fixture impedance and, therefore, can be neglected). Another example is that the transfer characteristic (the relationship between the voltage at the probing tip (input) and the output (e.g. the display)) of the measurement system is designed so that the voltage measured by the measurement system is exactly the same as (equal to) the voltage present at the point of probing tip contact (i.e. a perfectly flat frequency response). As the frequency of the voltages (signals) being measured increases, however, it becomes impossible to make a measurement system with impedance high enough so that the voltage at the point of probing tip contact of the DUT is unaffected by probing tip contact. Likewise, the transfer characteristic might vary with frequency, further degrading the accuracy of measurements taken with the measurement system. It is important to be able to accurately measure the effects of the impedance and the transfer characteristics (hereinafter referred to jointly as the “measurement system effects”) so that the accuracy of the measurement can be determined and/or improved through processing the measured data.
Many techniques exist to accurately measure insertable devices (e.g. devices that have standard male-female connecters that allow a test fixture to be inserted) as the DUT. When a measurement system (such as a Vector Network Analyzer (VNA)) is used to measure insertable devices, measurements are first made of several known standards, the most common being “short-open-load-through” (SOLT). From these measurements, the characteristics of the measuring system can be determined or calibrated. When the insertable device is then inserted between the test fixture ports (e.g. coaxial ports), the effects of the measurement system can be removed so that the DUT response alone can be displayed with no deviation due to measuring system imperfection.
The technique described above (as well as other known techniques in which multiple standards are applied) can be extended to situations that are not technically “insertable” such as assuming equal adapters or “Transverse ElectroMagnetic” (TEM) environments. (TEM environments have no electric or magnetic field in the direction of propagation.) Techniques used in these situations might include additional measurement standards (such as equal length thru paths or transmission line) or non-TEM calibration methods (such as TRL (thru-reflect-line)). All of these methods, however, require the connection between test fixture source port(s) (the signal source(s)) and test fixture response port(s) (the signal response(s)) to be broken (disconnected) so that different elements (e.g. different lengths of thru line or open-circuited transmission lines) can be inserted between the ports.
For purposes of this disclosure, measurement systems (including high impedance probes) are non-insertable and non-TEM devices. High impedance probes are designed to have a very high input impedance so that the signal (voltage) under test is not altered when the probing tip contacts the DUT being measured. These high impedance probes have traditionally been evaluated with specific test fixtures that interface with the probing tip to a fixed low impedance. For example, a test fixture usually has coaxial input ports facilitating measurement with a measurement system (e.g. a Vector Network Analyzer (VNA) or a time domain reflectometer (TDR)). The measurement system is calibrated to the reference plane of the test fixture input, and measurements are made of the high impedance probe and the test fixture. The test fixture is assumed to be perfect so that the resulting measurement is assumed to apply to the high impedance probe in parallel with the termination impedance (e.g. 50Ω). With a lower impedance test fixture, the impedance of the high impedance probe is assumed to be insignificant compared to the test fixture impedance and is neglected. Likewise, the test fixture is assumed to have perfectly flat frequency response. At high frequencies, however, neither of these assumptions is valid: the high impedance probe loading impedance alters the voltage present at the probing tip, and the transfer characteristic of the measurement system is dependant upon the specific implementation of the test fixture. At microwave frequencies the input or loading impedance of the high impedance probes can no longer be modeled as a simple parallel R-C circuit. In particular, since probe input impedance is designed to be much higher than 50Ω, the test fixture effects must be known precisely in order to accurately measure the small changes induced by the probe. A more detailed model is required that is valid throughout the operating frequency range of the high impedance probe. The probe impedance must be accurately measured throughout this frequency range in order to create this model.